“I fell in love with carrots because of smelling them in a root cellar,” says Chef Eric Skokan, owner of Black Cat Farm Table Bistro in Boulder, Colo. Skokan grew up on fresh produce stored in his grandparents’ root cellar, so when he needed a way to store the bounty from his farm during the winter, he built one—a square hole 6-feet deep, stairs to the bottom, with two doors that lie flat on the land.

From September to November, he leaves the doors open at night to get the cellar temperature to around 40F, which means his cellars (he now has two) rely completely on passive energy. “The technology is more than 2,000 years old,” he says.